The energies required to homolytically break the S-C, C-C, and C-Cl bonds in sulfur mustard, as well as the C-Cl bond heterolytically, are computed through a combined HF/6-31G* and MP2/6-31G* treatment. The effects of media of different dielectric constants epsilon are taken into account by means of the Onsager reaction field model. The homolytic bond-breaking energies decrease slightly as epsilon increases from 1.00 to 5.91 but then remain nearly constant to epsilon = 78.46. The energy required for heterolytic C-Cl cleavage is more affected by epsilon and also decreases rapidly as epsilon becomes larger.